Various hybrid powertrain architectures are known for managing the input and output torques of various prime-movers in hybrid vehicles, most commonly internal combustion engines and electric machines. Series hybrid architectures are generally characterized by an internal combustion engine driving an electric generator which in turn provides electrical power to an electric drivetrain and to a battery pack. The internal combustion engine in a series hybrid is not directly mechanically coupled to the drivetrain. The electric generator may also operate in a motoring mode to provide a starting function to the internal combustion engine, and the electric drivetrain may recapture vehicle braking energy by also operating in a generator mode to recharge the battery pack. Parallel hybrid architectures are generally characterized by an internal combustion engine and an electric motor which both have a direct mechanical coupling to the drivetrain. The drivetrain conventionally includes a shifting transmission to provide the necessary gear ratios for wide range operation.
Electrically variable transmissions (EVT) are known which provide for continuously variable speed ratios by combining features from both series and parallel hybrid powertrain architectures. EVTs are operable with a direct mechanical path between an internal combustion engine and a final drive unit thus enabling high transmission efficiency and application of lower cost and less massive motor hardware. EVTs are also operable with engine operation mechanically independent from the final drive or in various mechanical/electrical split contributions thereby enabling high-torque continuously variable speed ratios, electrically dominated launches, regenerative braking, engine off idling, and two-mode operation.
In multi-mode EVTs, as in conventional multi-ratio transmissions, torque transmitting devices may be employed in order to effectuate selection among various gear set speed ratios and direction control elements. Application and release of torque transmitting devices (commonly referred to as clutches), such as for transfer of torque between speed ranges in multi-range gearset arrangements, is known to be accomplished via the supply and exhaust, respectively, of high pressure hydraulic fluid to apply chambers of the torque transmitting devices. The apply chamber pressure acts on a piston which in turn causes the engagement of, for example, interdigitated friction plates or a band about a drum. Clutches may be stationary or rotating devices. Clutch application is conventionally controlled by way of a solenoid controlled hydraulic valve arrangement wherein transmission line pressure is modulated to the apply chamber to achieve a desired apply pressure. However, such systems are prone to single point failures such as stuck spool valves occasioned, for example, by manufacturing debris, contamination or wear particles carried by the hydraulic fluid.